Duplicates in SFDC will also be duplicates in Marketo. That means that the lead score and activity history is incomplete because it's spread out over multiple records. Essentially you lose the single view of the customer.
An alternative to creating duplicate leads is to create tasks for existing lead/contacts. You can assign these tasks to the lead/contact owners.
Alternatively, if you still would like to create duplicate Leads, maybe you can ensure that all of them get converted to Contacts, so that they are merged into a single record over time (Marketo will merge records if they are merged/converted in SFDC).
In addition to using SFDC web-to-lead, you can also use Marketo Webhooks to create duplicates. For example, you can use a Webhook to submit a web-to-lead form in SFDC: this will create a new lead in SFDC, which will then sync back into Marketo via the regular SFDC sync.